I. Field of the Invention
While the invention is particularly desirable for products with optical surfaces which are free from objectionable defects such as those caused by gate marks, it can be used generally for the molding of thermosetting and thermoplastic materials.
Optical quality products are in wide use. They are needed generally for assays where test substances are subjected to examination by electromagnetic radiation, including visible light. Optical products are also used for instruments, such as microscopes and ophthalmic devices.
Devices that require optical surfaces originally were prepared by grinding glass members. Such products now increasingly employ plastics to expedite manufacture and reduce cost. In general, the demand for plastic optical products is now considerably greater than for glass.
The shift from glass to plastic has occurred primarily because plastic is lighter and often has superior qualities. In addition, protective coatings to provide scratch and abrasion resistance for plastics have become available. Plastic also comes in a wide range of gradient-density tints and colors.
Of the many advantages exhibited by plastics, their relatively light weight and durability have proved to be significant. For optical surfaces, the lens thicknesses are the same for glass and plastic. Consequently, the reduced density of plastic produces a product that is of lighter weight.
The reduction in weight and density is particularly important when high powered surfaces are required, or when large optical surfaces are needed.
Previously, devices with large optical surfaces, particularly those of high power, were typically manufactured by the casting of thermoset resins, for example acrylics that were peroxide cured. However, the availability of polycarbonates and related thermoplastics permits the replacement of cast thermoset plastics. This is because modern polycarbonates have low densities and high refractive indices. For the same optical thicknesses, polycarbonates have an even lower weight than cast plastics, and far lower than glass.
Additionally, since polycarbonates have great impact strength and breakage resistance, they permit the production of relatively thin optical members. Moreover, coatings for polycarbonates are available to provide abrasion resistance. Polycarbonates are particularly suitable for products with xe2x80x9csinglexe2x80x9d optical surfaces, i.e., those with frontal convex and/or backside concave surfaces.
Optical surfaces are defined by two measures of the ray bending power of light or other waves. Spherical power produces magnification and/or reduction, while cylindrical power produces astigmatic corrections. The units of corrective power are in diopters. It often is desirable to have product available with a spherical power in the range from +4 (magnification) to xe2x88x926 (reduction) diopters, and a cylindrical power in the range from 0 to +2 diopters. Within this range, a volume-frequency distribution can be plotted, centered at zero power. There is reduced frequency in the plot as spherical or cylindrical power increases or decreases.
To be competetive, injection molded products require high yields, with a reduction in scrap and secondary operations, such as trimming.
Additionally, it is desirable to run optical surfaces of differing powers at the same time, without sacrificing productivity, quality or yields. A four-cavity moldset, for example, quadruples the productivity of a particular molding machine. Two of the cavities can be used to mold common spherical and cylindrical power combinations. The remining cavities can be used for less common surfaces.
An illustrative optical surface is found on an optical disk for the laser reading and storage of information. Optical disks for video respond to analog signaling, while compact digital disks are for audio signals. There also is a wide range of computer program disks for information and data storage. These include the CD/ROM (Compact Disk/Read-Only Memory) which is irreversibly encoded with program information, DRAW (Disk Read And Write, i.e. xe2x80x9cuser write oncexe2x80x9d) and EDRAW (Erasible, i.e. xe2x80x9cby the userxe2x80x9d, Disk Read And Write).
Many disks are encoded during molding by a xe2x80x9cstamperxe2x80x9d, which forms a face surface in the mold cavity. The digital information is represented on the stamper by a spiral of tiny projections, which, in turn, form indentations in the plastic molded disk. A typical indentation has a depth of 0.1 micron and a length of 1-3.3 microns, with a track pitch of 1.6 microns for a spiral array that extends radially outward.
One requirement of high quality molding is intimate contact of the polymer melt with the stamper, without any voids or premature shrinkage. Contact is maintained from the time the cavity is filled with melt until cooling takes place below the glass-transition temperature of the plastic.
Another requirement for optical products is reduction of internal stresses, i.e. xe2x80x9corientationxe2x80x9d, within the polymer. Ideally, the molding should be xe2x80x9cisotropicxe2x80x9d, i.e., exhibit the same properties in all directions, so that molded stresses and flow induced orientations are eliminated. Such stresses and orientations produce localized differences in ray bending power. The resultant nonuniformities in refractive index are measured in terms of optical path differences, commonly expressed as xe2x80x9cbirefringencexe2x80x9d. Avoidance of birefringence is desired.
With surfaces that employ laser signal reading, any flaw which disrupts or deflects the laser beam causes errors. Other properties which require consideration are percentage of light transmission, percentage haze, and the index of discoloration. Localized flaws include opaque specks or clear areas, such as voids or bubbles, which have different refractive indices and optical bending power than adjacent material. Absolute planarity or flatness are often needed where localized warpage would induce prismatic effects and result in off-axis signal transmissions.
In the molding of many optical surfaces it is necessary to conduct operations in clean or xe2x80x9cwhitexe2x80x9d rooms. Such rooms provide particle free environments in the range from Class 1,000 to Class 10. Since workers are the biggest source of contamination, automation of handling and post-molding operations is desirable.
Furthermore, for efficiency, microprocessor or CNC (computer numerical control) control should be used. The molding machines also should have individual moldsets, temperature controllers and hopper dryers. A clean air shower is needed for the clamp open and part removal position, together with robotic part pickers.
While optical molding commonly employs a single cavity mold, that makes inefficient use of clean room floor space, and results in a high captial and equipment fixed cost per part. Consequently, it is desirable to produce optical quality parts with multiple cavity molds.
Despite the advantages that have been achieved in the injection molding of optical quality products, there has remained the disadvantage of the product deformity associated with the need for gating the plastic into the mold. At the point of gating the resultant product invariably includes a blemish that requires removal in order for the product to be generally suitable. Typically the removal of the gate blemish takes place by polishing which requires a significant manual effort and additional cost.
II. The Prior Art
Early attempts to make acrylic or polycarbonate optical parts used injection molding with the mold cavity surfaces fixed throughout the molding cycle. This required long cycles, high mold surface temperatures approaching the glass-transition temperature of the plastic, along with high plastication and melt temperatures. Slow, controlled fill rates were followed by high packing pressures, which were held until the completion of gate, freeze-off.
Fixed cavity processes employ large gating and runner systems to permit appreciable packing pressure and delivery of material before gate freeze-off occurs. At that time no further transfer of molten polymer occurs. Gate freeze-off in fixed cavity injection presents a problem with surfaces having differing radii of curvature. It is the differences in curvature that produce the necessary ray bending needed for optical surfaces. Differing cross-sectional thicknesses result in non-uniform shrinkage during part formation and subsequent cooling. The thickest sections of optical parts are subject to sink marks or depressions which interrupt an otherwise uniform surface. This results in localized aberrations in ray bending character.
Even when care is taken that the injected polymer conforms to the surface of a fixed mold cavity, once gate freeze-off occurs, that prevents additional packing pressure and material transfer. This usually takes place in the thinnest cross-sectional area of the part, and differential shrinkage begins to occur within the melt. The polymer skin then pulls away from the mold surface, with greatest effect in the thickest cross sections. Pre-release, whether partial or complete, of the molded plastic before the cavity is unlocked and opened, detrimentally affects optical quality. The molded contours no longer provide precision surfaces.
Similar problems occur in the straight injection molding of parts with high aspect ratios, i.e., where there are relatively large surface dimensions and relatively small thicknesses. In those cases, a long length of flow is required through a small cross-sectional orifice of the mold cavity.
The most widely used polymers for the molding of parts with optical surfaces are polycarbonates and thermoplastic acrylics, particularly polymethyl methacrylate (PMMA).
Acrylics inherently have better flow at low melt temperatures, as well as low birefringence or polymer disorientation. However, they have relatively high water absorption which results in swelling and warpage, and relatively low creep resistance. Susceptibility to heat distortion make acylics less desirable, except for products, such as video disks, where parts are cemented together with encased information.
Polycarbonates, on the other hand, can have better performance, but are subject to serious processing limitations. Ordinary grades of polycarbonate have a low melt-flow index range, but higher melt-flow grades are available.
Even with high flow grades, the straight injection of polycarbonate causes high birefringence. This is because the mold cavity has fixed dimensions which do not change during the molding cycle, and exceed the finished part by a shrinkage compensation factor.
Polycarbonates are in amorphous chains that form random coils when in a relaxed state. When polycarbonate melt is forced through a restrictive flow path, or orifice, by high injection pressures the polymer distorts from stretching and shearing, realigning the polymer chains so that they are parallel to one another. This is believed to create severe anisotropy, i.e., nonuniformity. The incoming melt front can be regarded as a dynamically stretching zone of molten polymer. In this frontal zone, disorientation is caused by the shear of one polymer layer over another. This is a result of unavoidable velocity differences because the center of flow is faster than at the edges. In the resulting velocity profile, the lowest velocities are at the mold surface, and the highest velocity is at the center. A slowly moving melt front, at low pressure, produces a front that is less distorted and less stressed.
In straight injection molding of polycarbonates, injection is at the highest speed of the hottest, most fluid, polycarbonate melt into the narrow constrictions of a fixed cavity mold.
Elaborate plastication is needed to provide the hottest melt without catastrophic degradation in straight injection molding. Being less viscous, a hot melt provides less internal shear and slower freeze. This allows more time for melt relaxation after flow ceases, and before solidification. Such plastication can use starved feeding or a reduced sized barrel/screw combination. This minimizes the residence time of the polycarbonate polymer in the injection plastication unit, since high melt temperatures are required. Some plastications cause high shearing of the melt and suffer more polymer degradation.
The balance between degradation flawsxe2x80x94from a hot plastication meltxe2x80x94and high disorientationxe2x80x94from a fast fill rate into a high aspect ratio and restrictive mold cavity, creates a narrow xe2x80x9cprocess windowxe2x80x9d. This has made straight injection suitable only for single cavity molding. Multiple cavity straight injection would result in cavity imbalance.
Another difficulty with straight injection is that the contents of the mold cavity gradually shrink during cooling. This causes the part to pull away from the mold surfaces. Premature release can produce differential warpage or imprecise replication of surface contour patterns. Straight injection uses high injection pressures to maintain cavity pressure until gate freeze-off occurs. However, this application of pressure also causes re-extrusion or cold-flow of the increasingly viscous polymer core within the fixed dimensions of the mold cavity. Such forcible redistribution of the partially-solidifying melt creates internal stresses resulting in birefringence.
To overcome the difficulties associated with straight injection, resort has been made to mold cavity compression after injection. There are three types: (1) clamping with injection followed by compression, where compression is by platen motion; (2) auxilary component injection followed by compression, where there is full machine clamping with no platen motion, and mold-cavity compression is by auxilary components internal to the moldset; and (3) clamping and auxiliary component compression following injection, where mold cavity compression is by clamping and auxiliary component motion.
(C-1) Clamping With Injection Followed By Compression
Martin U.S. Pat. No. 2,938,232
As disclosed in Martin U.S. Pat. No. 2,938,232 (xe2x80x9cMartin ""252xe2x80x9d) for toggle-clamp injection molding, issued May 31, 1960 and known as a xe2x80x9csandwich pressxe2x80x9d, the mold platens and mold halves are brought together until a predetermined air gap is present at the parting line. At that point, a low pressure, low velocity injection fill begins.
After injection is completed and the molten polymer mass has cooled for a predetermined interval, the machine commences closure of the movable platen. This mechanically seals the mold cavity and its partially solidified contents with zero-clearance at the parting line. The mold halves are locked for the duration of the molding cycle at a predetermined clamp pressure. The partially solidified polymer mass is compressed by the amount of the air gap that existed at the parting line when injection started. By eliminating the air gap, the volume of the cavity and runner system is proportionately affected, resulting in compressive forces exerted upon the partially solidified polymer and causing a reorientation and re-flow. Under clamp induced compressive force, the mold cavity contents continue cooling and solidifying, eventually reaching a temperature sufficiently below the glass-transition temperature that the molded part may be ejected without optical distortion.
The result is clamp induced xe2x80x9ccoiningxe2x80x9d which offers advantages over straight injection. Successful coining is a function of initial injection pressure and fill rate, air gap dimensions, the timing interval between injection and compression, and the magnitude of the final clamping forces.
Control over injection pressure and fill rate, along with timing are critical. In order to prevent molten polymer from spilling outside the mold cavity, the injected melt must form a surface skin and partially solidify. Otherwise, molten polymer spills into the air-gap and necessitates trimming of the molded part.
If the melt has solidified excessively, compression at ultimate clamping pressures can cause deformation at the parting line and damage the moldset. The cooling interval is critical to achieving acceptable yields. If the melt is not sufficiently solidified at its most constrictive point, partially molten polymer can be extruded out of the cavity and into the runner system. This can result in an underfilled and underpacked part with badly distorted surfaces. However, if compression is delayed too long, too much polymer solidification will occur when the compressive force is initiated. This results in forceable reorientation of the polymer and xe2x80x9ccold workingxe2x80x9d of the plastic, producing birefringence and undesirable molded-in stresses.
Bartholdsten U.S. Pat. No. 4,409,169
To alleviate these problems of Martin ""232, Bartholdsten et al U.S. Pat. No. 4,409,169 teaches a slow, low-pressure injection of an oversized shot into a mold that is partially-open at the parting line, followed by deliberate melt cooling, viscosity thickening and a short pressing stroke to squeeze from the reduced mold cavity volume the partially cooled and viscous excess Aplastic. As pressing continues to the fully closed parting line position, radially extruded overl flow is pinched. Full clamping is maintained for shrinkage compensation and avoidance of prerelease.
Matsuda U.S. Pat. Nos. 4,442,061 and 4,519,763
Another clamp induced coining process is disclosed by Matsuda et al in U.S. Pat. Nos. 4,442,061 and 4,519,763. Melt is injected into a slightly opened moldset and cooled until fully solidfied. The melt is then reheated uniformly above the melt temperature, at which point a clamp actuated compressive stroke is delivered and maintained throughout a second cooling cycle.
(C-2) Auxiliary Component Injection Followed By Compression
Another type of injection followed by compression molding makes use of auxiliary components, such as springs or cylinders to apply compressive force to internal and opposing mold surfaces. The primary difference over clamping injection/compression is that mold compression is provided by a stroke producing element, whereas mold compression in auxiliary componentxe2x80x9d molding is provided by auxiliary springs or hydraulic cylinders. Furthermore, clamping injection followed by compression is sequenced and coordinated by process control, while auxiliary component compression is controlled by self-action, like springs, or separately by timers.
A further distinction is that auxiliary component compression does not employ the motion of a movable platen to provide compressive forces to reduce cavity volume. Instead the mold is fully clamped with no relative motion of the clamp plates, or of fixed and movable platens, during the injection fill, cavity reduction compression, or cooling.
Examples of auxiliary component injection followed by compression are discussed below.
Johnson U.S. Pat. No. 2,443,286
In Johnson U.S. Pat. No. 2,443,286, issued Jun. 22, 1948, spring loaded, movable dies are employed within the moldset. This creates a variable volume mold cavity, but relies upon high internal polymer melt pressure to spread the movable dies against resisting spring pressure. In order to apply a sufficiently great compressive force to the solidifying contents, substantial spring forces are needed. However, the greater the spring force, the greater the injection pressure needed to compress the springs during variable cavity fill. The greater the injection pressure, the greater the degree of molded-in stress and unsatisfactory birefringence. This type of process generally is limited to production of weak optics with small surfaces and limited thickness.
Weber U.S. Pat. Nos. 4,008,031 and 4,091,057
Another auxiliary component process is disclosed in Weber U.S. Pat. Nos. 4,008,031 and 4,091,057. A variable volume cavity is formed by injection melt and by the pressure induced rearward deflection of at least one movable die. After an interval, forward displacement results in compression under the driving force of an auxiliary hydraulic cylinder mounted in a one-to-one relationship with the movable die. Flow ports are provided for excess, increasingly viscous and partially cooled injected polymer melt which is extruded from the cavity under compressive forces.
Weber teaches slow mold fill, and, as with conventional clamp induced coining, relies upon a preset lapse of time between completion of injection fill and commencement of compressive pressure. Accordingly, Weber is faced with the problems of premature compression, i.e., inadequate solidification, or delayed compression, i.e.,late solidification.
In addition, Weber can produce inconsistent parts with variable thicknesses. Depending upon the timing interval, the travel of the movable die is controlled by the length of time elapsing after molten plastic enters the variable cavity and pressure is applied to the movable die. The final volume of the cavity also is controlled by the time elapsing after molten plastic enters the variable cavity, and by the length of time that pressure is applied to the movable die. The result is product variation within the same production run, and thickness variations.
Moreover, when Weber employs a two cavity mold the compression of each cavity is controlled by a separate and independent hydraulic cylinder. Consequently, the two cavities are not simultaneously acted upon by a common component. The larger the number of cavities, the larger the expected variations.
Laliberte U.S. Pat. No. 4,364,878
Another auxiliary component process is disclosed by Laliberte in U.S. Pat. No. 4,364,878. Laliberte includes a movable die coupled to an auxiliary hydraulic cylinder. After the mold is closed under clamp pressure, the mating die parts are spread apart. A precise, volumetrically metered shot that is just adequate to fill the fully-compressed mold cavity is then injected. This control of shot size allows compression without displacement of partly solidified melt out of the mold cavity through an overflow port. The result is greater control over part thickness, eliminating scrap waste and trimming.
However, Laliberte is limited to one-cavity production by reliance upon precisely metered melt, corresponding one-to-one with the injected melt. In addition, there is dependence upon an individually controlled and sequenceable hydraulic cylinder in a one-to-one motion relationship with a variable volume cavity.
While auxiliary component processes have to some degree been useful in molding optical surfaces, they cannot be applied generally.
Compressive forces for auxiliary component molding are much less than those available through clamp actuated coining. This limitation is particularly troublesome for optical surfaces with large projected areas and the necessity for initimate contact with the melt.
(C-3) Clamp and Auxiliary Component Injection Followed By Compression
Maus and Galic U.S. Pat. No. 4,828,769
Another prior art technique with clamp and auxiliary component injection followed by compression is disclosed in U.S. Pat. No. 4,828,769 which issued May 8, 1989 to Steven M. Maus and George J. Galic. According to this teaching, an article is formed from a plasticized thermoplastic resin using an injection molding machine in which an opposing pair of mold inserts are initially separated to form a pre-enlarged cavity.
A mass of plasticized resin, slightly larger than the volume of the article to be formed, is injected into the mold cavity. The main clamp force of the injection molding machine is initiated, shortly before completion of the injection to overcome inertial effects. After the completion of injection, compression begins and the clamping reduces the volume of the closed mold cavity in order to redistribute the resin. The main clamp force is applied until a final clamp lock position is reached.
In addition, the molding machine has first and second mold platens, first and second parting line mold plates, a plurality of first mold inserts operatively disposed within a mold plate forming a first parting line, and a plurality of second mold inserts operatively disposed within a mold plate forming a second parting line. The first and second mold plates, and the first and second mold inserts, are respectivly commonly supported by the first and second mold platens. The mold plates initially are urged together to eliminate any parting line air gap.
In all of the prior art discussed above, the product has been accompanied by inevitable gating marks. These marks not only produce an undesirable disfiguration of the product, they interfere with its optical and radiation transmission performance. To date there has been no successful technique for the elimination of gating marks, whether the gating deformitites were produced by a cold runner system with a subgate leading from the cold runner to the mold cavity, or whether by a hot runnner system in which a melt stream is applied to the cavity through a short thin neck that hardens when the part is formed. In all cases the removal of the part from a cavity entails the severance from the short thin neck of the runner system or the severance from the subgate connection of the cold runner system.
III. Objects of the Invention
Accordingly it is an object of the invention to expedite the molding of articles, particularly those which have optical surfaces.
Another object is to overcome the difficulties associated with prior art straight injection, clamping injection, and auxiliary component injection, and injection-compression parting line molding where compression of melt takes place after injection is completed.
A further object of the invention is to eliminate the deformities and defects associated with conventional gating.
In accomplishing the foregoing and related objects, the invention provides a method of forming a gateless article from a molten plasticized resin using a molding machine which forms a mold cavity having a first region for producing a molded part and a second region for receiving a plasticized resin. A volume of plasticized resin is injected into the second region for the article to be formed in the first region of the mold cavity. Pressure is then applied to the plasticized resin to move the resin at least in part from the second region to the first region. Pressure is maintained on the plasticized resin in the first region until the article is formed and stresses are relieved.
In accordance with one aspect of the invention, the first and second regions of the mold cavity are formed with respect to a movable mold insert and/or a mold member for receiving the insert. Pressurization takes place with or after the completed injection of plasticized resin into the mold cavity to reduce the volume thereof to the first region and fill the first region while driving gasses therefrom. The insert is displaced within its mold member while the mold cavity is closed and is displaced below the second region during the injection of the plasticized resin.
The article is formed with a boundary at the parting line or interface between mold parts. The injection of the plasticized resin takes place through a plurality of orifices which direct the plasticized resin into the second region.
The machine advantageously has fixed and movable platens mounting the insert and an opposing mold member, at least one of which is capable of movement relative to the other. The insert is initially separated within its mold member to form a pre-enlarged cavity for receiving resin with reduced back pressure. The injected resin is through a sidewall of a mold member housing a movable insert and through a subgate into said second region of the mold cavity and through the neck of a hot runner system.
Apparatus for forming a gateless article from a molten plasticized resin includes members for forming a mold cavity, with a first region for producing a molded part and a second, distinctive region for receiving a plasticized resin. Injection into the second region is of a volume of plasticized resin for the article to be formed in the first region of the mold cavity. Pressure is applied to the plasticized resin to move the resin at least in part from the second region to the first regions, and maintained on the plasticized resin in the first region until the article is formed and stresses are relieved.
The cavity can be formed by a first mold member affixed to a first mold platen, and a second mold member with an internal, movable insert affixed to a second mold platen. The insert is surrounded by a movable mold member which engages the first mold member to form the first and second mold cavity regions. A main clamp force controls the position and velocity of the insert relative to the mold member. The insert has an initial stroke to close the gate by which resin is injected into the mold, followed by a stroke beyond the gate to compress the resin and compensate for cooling-induced shrinkage. A gate for the injection of resin into the mold extends along the parting line between the mold members.
The invention also is directed to simultaneous injection and pressurization molding which is particularly useful in producing, for example, optical quality products at high output yields and reduced cost.
One method includes forming an article from a plasticized resin by an injection molding machine that uses a die with a moveable insert. A closed mold cavity is formed for receiving plasticized resin. A mass of plasticized resin, equal or larger than the mass of the article to be formed, is injected into the mold cavity. Simultaneously with the injection of melt, the insert is withdrawn to increase the volume of the closed mold cavity and permit the introduction of melt without introducing significant backpressure.
The invention also includes the formation of a multiplicity of articles simultaneously from plasticized resin.
In accordance with one aspect of the invention, a homogeneously plasticized polymer, such as a polycarbonate, acrylic, polymer styrene, ABS (acrylic styrene) or EKTAR (a Kodak copolymer of polycarbonate and styrene), is prepared in a reciprocating screw injection molding machine that is equipped with process controls, preferably an open or closed loop microprocessor, or conventional molding machine controller.
A reciprocating screw delivers a precisely-metered volume of melt, greater than or equal to the total mass needed to form parts, for example, by a runner system connected in fluid communication with mold cavities and a nozzle source.
In accordance with another aspect of the invention, a variable volume cavity is created by insert movement. During injection fill from a plurality of injection ports, the volume of the cavity is progressively increased until the volume is greater than the volume of the resulting finished article. The volume of the injected mass also is greater than that of the finished article to compensate, upon completion of cooling, for shrinkage and subsequent demolding.
In accordance with another aspect, the injection of resin occurs at a relatively high fill rate, but, because of the variable volume cavity, the injection is at reduced pressure at entry points of the cavity. This reduces resulting molded-in stresses and internal strains in the molded part.
In accordance with a further aspect, the pressurization of the melt is initiated with the insert retracted. The stroke can be profiled using sensors, such as those associated with screw position, even before the screw has actually begun its travel, and before and during subsequent full delivery of the predetermined injection shot size. Early sensing, even before injections, compensates for the inherent inertia in commencing actual pressurization travel of the movable platen.
The melt pressurization is monitored in accordance with further motion of the movable platen, but other machine elements, such as the ejector, could be used instead. Changes in position and velocity are determined by numerically controlled clamp profiling, for example using an opened or closed loop microporcessor, or conventional molding machine process controller. The pressurization can be xe2x80x9csingle stagexe2x80x9d in which increasing back pressure of the melt progressively slows the pressurization stroke. Preferably the pressurization is multi-stage, with at least one relatively fast phase and one relatively slower phase. The faster phase helps displace any void volume or gas in the oversized cavity and quickly commences pressurized redistribution of the relatively hot, minimally solidified thermoplastic mold cavity contents into its preferred isotropic orientation. This assures intimate contact with precisely polished mold surfaces, to produce the desired molded part configuration. The last stage of the multi-stage pressurization proceeds at a comparatively slower travel and is largely used to maintain intimate contact between the cooling and shrinking polymer mass and the mold surfaces to avoid prerelease and optical distortions.
A multistage pressurization can include any number of intermediate steps, but preferably includes at least one relatively fast travel step, followed by a relatively slower travel step. This is to maintain molded part shrinkage control throughout the cooling process until the the molded part is well below its glass transition temperature and ready to be ejected.
In accordance with still another aspect of the invention, following multistage pressurization and gradual cooling of the polymer, solidification and cooling are completed to the point where the molded parts can be ejected from the mold. At that time the core is retracted to its fully withdrawn position and conventional ejection is employed. Meanwhile, during cooling, the plasticating screw is prepared and metering takes place for the next shot to be delivered on the next molding cycle.
Where the same driving force is used for pressurization, all cavities in a multicavity mold will receive the same pressurization forces at the same time.
Cavity filling desirably takes place at low injection pressures, but the process is not dependent upon commencing pressurization at a preset time interval, with an inherently high level of error, but rather pressurization is determined with comparative precision by sensing a digitally settable screw position using a controller.
The method of the invention also forms a gateless article from a material using a molding machine by (a) forming a mold cavity having a first region for producing a molded part and a second, distinctive region, including a gate, for receiving the material; (b) injecting into the second region through the gate a volume of the material for the article to be formed in the first region of the mold cavity; and (c) applying pressure to the material to move it beyond the gate from the second region into the first region so that the article does not have any gating marks.
Force is maintained on said the members until the article is completely formed without gating marks.
The volume of the second region can be controlled by a movable insert of the machine, and the insert can be displaced within the mold cavity while it is completely closed, for example, displaced below the gate during the injection of the material, which can be injected through a plurality of gates.
The gates can terminate in diagonal channels for the injection of the material, and the mold can have a parting line in the first region. The injection can be through a sidewall of the mold cavity and be injected through a an edge-gate for a hot runner, or through the neck of a hot runner, with the material being a plasticized resin.